Coupling photocatalytic CO2 reduction and CH3OH oxidation for selective dimethoxymethane production

Currently, conventional dimethoxymethane synthesis methods are environmentally unfriendly. Here, we report a photo-redox catalysis system to generate dimethoxymethane using a silver and tungsten co-modified blue titanium dioxide catalyst (Ag.W-BTO) by coupling CO2 reduction and CH3OH oxidation under mild conditions. The Ag.W-BTO structure and its electron and hole transfer are comprehensively investigated by combining advanced characterizations and theoretical studies. Strikingly, Ag.W-BTO achieve a record photocatalytic activity of 5702.49 µmol g−1 with 92.08% dimethoxymethane selectivity in 9 h of ultraviolet-visible irradiation without sacrificial agents. Systematic isotope labeling experiments, in-situ diffuse reflectance infrared Fourier-transform analysis, and theoretical calculations reveal that the Ag and W species respectively catalyze CO2 conversion to *CH2O and CH3OH oxidation to *CH3O. Subsequently, an asymmetric carbon-oxygen coupling process between these two crucial intermediates produces dimethoxymethane. This work presents a CO2 photocatalytic reduction system for multi-carbon production to meet the objectives of sustainable economic development and carbon neutrality.

standard process.The spectra were calibrated, averaged, pre-edge background subtracted, and post-edge normalized using the Athena program.The Fourier transformation of EXAFS oscillations from k to R space was achieved to obtain a radial distribution function.All data fitting was completed in the Artemis program.The 300 W Xenon lamp (320-780 nm, PLS-SXE300, Beijing Perfectlight Technology Co., Ltd) provided the UV-visible light source.The BET (Brunauer-Emmett-Teller) and HK (Horvath-Kawazoe) methods were used to determine the specific surface area and micropore analysis, respectively, by using BELSORP-max through liquid N2 cryo-sorption (Micrometrics ASAP2020, USA).Photoluminescence (PL) spectra were obtained using an Agilent Cary Eclipse Fluorescence spectrometer.The UV-Vis-diffuse reflectance spectra (DRS) were recorded on the Agilent Cary 7000.The electrochemical photocurrent and impedance (EIS) were obtained using an electrochemical workstation (CHI-660E, USA) with a three-electrode system.For working electrode preparation, 5 mg of catalyst and 20 μL of Nafion solution (5%) were dispersed into a 230 μL mixture solution including DI water (100 uL) and IPA (isopropanol, 130 uL) by sonication for 30 min.250 μL of suspension was dropped onto the conductive side of FTO glass with the size of 1×1cm 2 .After the sample is thoroughly dried at room temperature on the FTO glass, the mass loading of all catalysts is determined as 5 mg cm -2 .The Ag/AgCl electrode and Pt mash were used as the reference electrode, and counter electrode, respectively.The 15 mL 0.1 M Na2SO4 with 5 mL CH3OH aqueous solution is the electrolyte.Before the electrochemical test, the CO2 gas flowed in the electrolyte for 30 min then kept the system closed.The productions after the CO2 reduction reaction were detected by the GC system (7890A, Agilent Technologies, USA) with porapak N and molecular sieve column. 13C labeling experiments were measured by Gas Chromatograph Mass Spectrometer (GC-MS) system (YL 6900, YL instrument CO., LTD.Korea) by column-Agilent PoraPLOT Q under temp.150 o C 35 ℃/min.The femtosecond transient absorption spectroscopy (fs-TA) was measured using a TR spectrometer (Helios, Ultrafast Systems) operating at a center wavelength of 350 nm with a bandpass filter, one operated by an optical parametric amplifier (TOPAS Prime, Light Conversion), which was used as the pump beam.The wavelength of the probe is 370-640 nm, and the pump power is about 200 nJ.The surface temperatures of the Ag.W-BTO were assessed using an infrared thermal imager (Teledyne FLIR, FLIR TG165).25 mg of photocatalyst was positioned at a distance of 40 cm from the 300 W Xenon lamp cap (wavelength, 320-780 nm).

Band gap determination from UV-vis diffuse reflectance spectra
The UV-vis diffuse reflectance spectra were converted to absorption spectra using the Kubelka-Munk equation (1) first,

2R
(1) where F(R) and R represent the absorption coefficient and the relative reflectance of samples with infinite thickness in comparison to the reference, respectively.Furthermore, the band gaps of samples were estimated using the Tauc equation (2), in which h, ν, A, and Eg represent the Planck constant, light frequency, proportionality constant, and optical band gap, respectively, while n is determined by the nature of the transition in a semiconductor.Values of 1, 3, 4, and 6 for n correspond to the allowed direct, forbidden direct, allowed indirect, and forbidden indirect transitions, respectively.The values of Eg were calculated from the plot of (F(R)hν) 2/n against hν and corresponded to the intercept of the extrapolated linear portion of the plot near the band edge with the hν axis.BTO-related samples were treated as semiconductors with allowed indirect transition 1 .

Apparent quantum yield (AQY) calculation method
In terms of AQY, the Xe lamp was also replaced by the monochromatic LED light (CEL-LEDS35, Beijing Perfectlight Technology Co., Ltd, the wavelengths (λ) are 395, 420, 500, and 595 nm, respectively).Other experimental parameters are the same as the photocatalytic DMM production process.The light intensity was monitored by an optical power meter (CEL−NP2000−2, Beijing Perfectlight Technology Co., Ltd).The number of incident photons (N) is calculated by equation ( 3), and AQY is then calculated in equation ( 4).

DFT calculation method
All calculations were implemented using the Vienna Ab initio Simulation Package (VASP), based on DFT.We have employed the first principles 2,3 to perform DFT calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) 4 formulation.We have chosen the projected augmented wave (PAW) potentials 5,6 to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 520 eV.The GGA + U method was adopted in our calculations.The value of the effective Hubbard U was set as 4.814 eV for Ti.Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV.The electronic energy was considered self-consistent when the energy change was less than 10 −5 eV.A geometry optimization was considered convergent when the energy change was smaller than 0.05 eV Å −1 .
In our structure, the U correction is used for Ni atoms.The Brillouin zone integration is performed using 2×2×1 Monkhorst-Pack k-point sampling for a structure.Finally, the adsorption energies (Eads) were calculated as: Eads= Ead/sub -Ead -Esub (5)     where Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively.